1. Field of the Invention
The present invention relates to a measurement method and system. More particularly, the present invention is directed to an optical and mechanical scanning system for the capture of high-speed images.
2. Description of Related Art
High-speed imaging has a history filled with creative solutions for capturing ultra-short events with high spatial resolution, high dynamic range, and high temporal resolution. The most common methods over the years involve rotating mirrors (which quickly scan an image of the object to different regions of stationary film) or spinning drums (which rotate film at high-speed relative to a static image). Rotating mirrors are usually made of beryllium and are severely stressed by centripetal forces (from the high angular velocity) to the point that failures can and have occurred. Even if the exploding minor can be contained, beryllium dust is known to cause dangerous diseases when inhaled. Rotating mirror cameras, in addition to being dangerous, are expensive to purchase and operate, and have severe limitations in both temporal and spatial resolution.
There exist electronic methods to capture multiple images in which electrons in a CCD imaging camera chip are transferred from an exposed region to a masked region (interline transfer, frame transfer, etc.). However, this suffers from low resolution and/or low number of frames because all the image frames must share the real estate of one semiconductor chip (for example, a 16-megapixel CCD could store sixteen 1-megapixel images, or four 4-megapixel images, etc.). The Japanese company Shimadzu is marketing a 100 frame camera (for sports applications) in which each frame only has 312 by 260 pixels. Such cameras also suffer from poor dynamic range (for example, the Shimadzu camera provides less than 8 bits of grayscale, while film on the other hand can provide well over 16-bits (hundreds of times better dynamic range).
Yet another image capture method uses the Shaw Camera or Image Conversion Camera which gates an image of electrons (generated by an optical image upon a photocathode) by briefly accelerating these electrons with a large electric field (from a shaft high voltage pulse) toward a phosphor screen, wherein the image of electrons is converted back into an optical image (through luminescence) and recorded on film or a semiconductor detector array (CMOS, CCD, etc.). It does not matter that the luminescence can be slow, continuing to expose the detector after the gate is off, because the source image of electrons ceased when the high voltage was removed. The photocathode, however, must have a fast response to prevent integration “blurring” of the image which is quickly changing over time. One major disadvantage of this method includes the requirement for an electronic gate (proxifier or microchannel plate) and a detector array per frame. The cost is further increased by a low-jitter, high-voltage pulser required for each gate.
Multiplexed holography has also been employed for the capture of a series of brief exposures. As many as 10,000 images have been angle multiplexed into a single storage medium. The short exposure (to freeze motion) is controlled by using short and bright laser pulses. Holography, however, requires a rather special laser which has a single wavelength (typically a cavity-seeded, single longitudinal mode) and single transverse mode (TEM00). Companies like InPhase Technologies are hoping to store a terabyte of data on a single CD ROM by employing various holographic multiplexing schemes and utilizing the inherently high throughput of writing two dimensional binary (not grayscale) images (with a spatial light modulator) and reading out the data one image at a time (with a two dimensional digital camera system).
Accordingly, a need exists for methods and apparatus to make high speed imaging systems more accessible to scientists, engineers and the general public through a system that is lower in cost, safer to operate, covers a longer event window (more image frames), and has higher dynamic range and higher spatial and temporal resolution. The present invention is directed to such a need.